The present invention relates to a system and method for controlling the driving force of a motor vehicle, such as an automobile. More particularly, the invention relates to a system for maintaining motor vehicle driving force by controlling the rotational speed of the driving wheels to minimize slipping. The system is applicable to automobiles, trucks, tractors, and the like.
In conventional systems for controlling the driving force of a motor vehicle, a control characteristic value of the driving wheels, such as the slipping ratio or the rotational speed, is compared with a threshold level determined in accordance with the rotational speed of one or more of the driven wheels. The system controls a brake force applied to the driving wheels so that the control characteristic is maintained at a constant relationship with respect to the threshold level.
In the initial start of the vehicle on a slippery or a slithery road, however, the rotational speed of the driven wheels is extremely low while that of the driving wheels is high, and the variation in the difference in rotational speed between the driving and the driven wheels may be large. In other words, the slipping ratio is extremely large and may rapidly change. Accordingly, it is difficult to precisely control the brake system and to obtain a predetermined driving force when the slipping ratio is employed as the control characteristic in the low-speed range of the motor vehicle.
On the other hand, when the motor vehicle is in a high-speed range, the variation in the slipping ratio would be minimized as a variation of the rotational speed of the driving wheels relative to the driven wheels is minimized. During high-speed operation of the vehicle, since it is required to obtain a driving force with a high efficiency, that is, a low fuel-consumption rate, it is desirable to employ a slipping ratio as the control characteristic value which directly affects the driving force, rather than the rotational speed of the wheels, to control the system.